Influenza is a highly contagious respiratory disease that causes substantial mortality world-wide. There is great international concern that the Avian Influenza virus H5N1 will acquire the ability to transmit between humans and cause a global catastrophe. Consequently, the World Health Organization and the US National Strategy for Pandemic Influenza have delineated goals for stockpiling drugs that can protect against influenza infection. However, currently available H5N1 vaccines are poorly immunogenic and fail to induce broad cross-protective antibody responses to drifted viruses. Therefore, there is an urgent need for alternative treatments that provide potent and broadly cross-protective host immunity that can rapidly neutralize and control dissemination of H5N1 virus in humans. This Phase I SBIR proposal outlines a novel strategy that will simplify and accelerate monoclonal antibody (MAb) drug discovery for Flu. Our methodologies are designed to (1) augment induction of cross-reactive neutralizing antibodies in vivo and (2) permit rational selection of anti-hemagluttinin immunoglobulin (Ig) variable domains for building therapeutic monoclonal antibodies. Our novel approach combines an immunization protocol that dramatically increases antibody diversity in mice with DNA sequencing tools that provide a deep survey of the resulting Ig variable domain repertoire. We propose the following aims: (1) Select and characterize heterosubtypic neutralizing anti-H5 antibodies, and (2) Identify a lead antibody that is cross-protective in animal models of influenza infection. This Phase I SBIR proposal will establish proof of concept that cross-reactive anti-HA MAbs can be selected from immunized animals using a sequence-first screening strategy and that they are effective in advanced animal models of H5N1 infection. Successful completion of this project will result in a small set of therapeutic chimeric antibodies and will provide a strong rationale for conducting the necessary IND-enabling studies in a subsequent Phase II proposal. Clinically, these antibodies will impact the treatment and prevention of pandemic flu. Scientifically, the availability of these antibodies will clarify HA structure-function relationships, lead to the identification of new epitopes for vaccine development, and guide the regulation of adaptive B cell responses, possibly in humans. We also believe that the concepts, methods and technologies described in this proposal will be broadly applicable for developing therapeutic antibodies to a variety of infectious diseases.